The invention relates to a decoding process and apparatus in which a cipher transmitted with recurring phase-in information is decoded by detecting the phase-in information and generating a key pulse sequence related to the phase-in information, and more particularly, to a novel decoding process and apparatus which permits the decoding of parts of the cipher received before detection of the phase-in information.
In high frequency (HF) message transmission, frequently a so-called broadcast operation is customary when transmitting a certain message from a control station to a series of receivers. Such a receiver is often unable to request, by means of call-backs, a repetition of the messages in the case of messages received with interference or in fragments only, for example, because as a pure receiver it does not have an emitter, or because it must not break radio silence for tactical reasons.
By using error correcting codes in such cases it is often possible to improve the likelihood of successful transmission but many of these codes--particularly the blocking codes--require synchronization, which the receiver must first establish. The same is true for coding, wherein the emitting and receiving stations must first be brought into step by an initial synchronization. Should the receiver be unable to effect this, no decoding is possible. Generally, in such cases the receiver must receive two items of information correctly: the cryptoinformation and the starting time. The first may consist of an auxiliary key, a coded message key or the like, and its purpose is to effect the automatic changing of the code program for each transmission; the second defines the starting time and thus the crypto-synchronization. The crypto-information and starting time together are generally designated the so-called entry or phase-in information.
In keeping with its importance, this entry or phase-in information is transmitted redundantly at the start of the message so that in spite of a certain number of transmission errors decoding is still possible.
However, particularly in the case of HF transmission, such safeguarding is still unsatisfactory, because as the result of fading or strong interference the phase-in information may be completely lost; the effect is then the same as if the receiver would have been actuated too late.
A substantially better performance is obtained if the phase-in information is repeated during the transmission periodically or aperiodically, in certain intervals. The use of such a procedure is described for example in the article "9EC755 Digital Voice CipherSystem for Defense Applications" by G. Baltzer in Philips Telecommunications Review, March 1981. If as the result of strong interference, for example, the first three transmissions of the phase-in information, the so-called late entry points, are missed, synchronization may be accomplished the next time and at least the remaining part of the message decoded. Once synchronization is established, it is frequently possible to hold during any later interference, provided that the generators on both the emitting and the receiving side are operating continuously and the existing phase-in information defines an instantaneous position (in the coded form). This provides the receiver with the opportunity to retest its correct synchronism at each late entry point.
The disadvantage of this system is obvious: the parts of the message arriving prior to synchronization cannot be decoded and are thus lost. As is very frequently the case, with tactical reports, for example, the correct reception of even the smallest part of a report may be extremely important, this disadvantage is often problematic and severe.